1. Field of the Invention
The present invention relates to an information recording and reproducing unit using a giant magnetoresistance sensor (hereafter referred to as GMR sensor), particularly to recovering from errors in the performance of the GMR sensor in the information recording and reproducing unit.
2. Description of Related Art
An MR sensor using the anisotropic magnetoresistance effect has been used as the reproducing head of a magnetic disk drive. However, a GMR sensor has been recently used which is superior in reproducing capacity. The GMR sensor shows a high magnetoresistance change to a low magnetic field stored in a medium by using the giant magnetoresistance effect and realizes a high recording density. A GMR sensor is described in IEEE TRANSACTIONS ON MAGNETICS, Vol. 30, No. 6 November 1994, xe2x80x9cDesign, Fabrication and Testing of Spin-Valve Read Heads for High Density Recordingxe2x80x9d.
In general, a GMR sensor is constituted by arranging two ferromagnetic layers at the sides of a non-ferromagnetic layer. One ferromagnetic layer is referred to as a free layer and the magnetizing direction of the layer is oriented in a certain direction when there is no external magnetic field. However, when an external magnetic field is applied to the layer, the layer can freely rotate in the direction of the applied magnetic-field. The other ferromagnetic layer is referred to as a pinning layer and the magnetizing direction of the layer is fixed to a direction perpendicular to the magnetizing direction of the free layer in the state of no external applied magnetic field, and it is not changed due to an external magnetic field. To fix the magnetizing direction of the pinning layer, an anti-ferromagnetic layer referred to as a fixed layer is attached so as to directly contact the pinning layer. The fixed layer provides a fixed magnetic field in order to pin the magnetizing direction of the pinning layer through switched connection.
When making a sensor approach the surface of a disk, magnetizing directions of a free layer are changed in accordance with the direction of the magnetic flux on the disk surface and electric resistances of the sensor are also changed. Therefore, by supplying a bias current to the sensor, it is possible to read the data stored in the disk as the change of magnetizing directions.
To magnetize an anti-ferromagnetic layer (fixed layer) in a predetermined direction, the anti-ferromagnetic layer is heated up to the blocking temperature or higher and cooled in a magnetic field suitable for pinning. The blocking temperature is a temperature at which the exchange anisotropy of the anti-ferromagnetic layer disappears.
When the temperature of a GMR sensor used for a magnetic disk drive undergoes a temperature stress for a long time due to temperature rise in the disk drive, physical contact between the GMR sensor and a magnetic disk serving as a storage medium, and heating by a bias current, a pinning layer cannot be pinned in the initial direction due to surrounding magnetic fields and the amplification factor lowers or a regenerative waveform is deformed and thereby, a predetermined performance cannot be displayed. Thus, an operation error occurs relating to a read error or user data or servo data.
U.S. Pat. No. 5,650,887 commonly assigned with the present application discloses a system for detecting the magnetizing direction of the fixed layer of a GMR sensor used for a magnetic disk drive when the direction deviates from the predetermined direction and recovering the direction by applying a predetermined current pulse. In the case of this patent invention, it is decided whether a read error arising in user data is caused by a read error of servo data and the amplification factor of an automatic gain control circuit is a predetermined value or more and thereafter, a reset pulse is sent to the GMR sensor. Moreover, after applying the reset pulse, the normal error recovery operation is started. Part of the reset pulse heats the GMR sensor up to the blocking temperature or higher and another part for lowering the temperature of the GMR sensor up to the blocking temperature or lower and applying a magnetic field in a predetermined direction.
The performance of a GMR sensor set to the actuator arm of a magnetic disk drive can be known through an operation error indication of the disk drive under working conditions. However, an operation errors can occur due to causes other than deterioration of the performance of the GMR sensor, e.g. swinging of the spindle or vibration of the disk drive. An error due to deterioration of the performance of the GMR sensor is distinguished in the disk drive from an error due to other causes and processed by an error recovery program (ERP). If a current or voltage reset pulse is applied to a GMR sensor whose performance is not deteriorated to perform GMR resetting, the performance of the GMR sensor may be deteriorated. Moreover, unless the performance is recovered by one-time application of a reset pulse, it is possible to recover the performance by gradually increasing the amplitude of the reset pulse and repeating GMR resetting several times. However, even if further applying a reset pulse to a GMR sensor whose performance is deteriorated due to application of a reset pulse, the performance cannot be recovered in most cases. When increasing the amplitude of a reset pulse and continuously applying the reset pulse to a GMR sensor whose performance is not deteriorated, the GMR sensor is damaged.
When the performance of a GMR sensor is deteriorated, an ERP recovers an error by reassigning a sector from which data can barely be read or in which data cannot be written to a substitute spare sector. However, the number of substitute sectors is limited. Therefore, if every substitute sector is used, subsequent errors cannot be recovered by reassignment. Moreover, it is preferable to not use a substitute sector because use of the substitute sector lowers the access speed of a head to a magnetic disk.
Furthermore, because a storage medium of a magnetic disk drive generates a magnetic field on its surface, the GMR resetting effect cannot be completely obtained if a GMR sensor is strongly influenced by the magnetic field immediately after a reset pulse is applied to the sensor.
Therefore, it is an object of the present invention to improve the performance and reliability of an information recording and reproducing unit such as a magnetic disk drive using a GMR sensor.
Specifically, it is an object of the present invention to decide whether an error unrecoverable by conventional ERP arising in an information recording and reproducing unit is caused by a GMR sensor before performing GMR resetting, improve the effectiveness of GMR resetting without accelerating the deterioration of the sensor by careless GMR resetting, and quickly recover the error.
It is another object of the present invention to decide whether the last resetting deteriorated the performance of a sensor relative to the first resetting and the performance of the head after resetting performed before the scheduled resetting in order to perform GMR resetting at the second time downward and thereby, determine whether to perform the scheduled resetting, improve the certainty, and prevent a GMR sensor from being damaged.
It is still another object of the present invention to count the number of reassignments of a sector due to a read or write error even when an error can be recovered by an ERP, prevent the access time of a head due to reassignment from increasing by resetting the head in accordance with the number of reassignments for each head, and prevent every substitute sector for reassignment from being used.
It is still another object of the present invention to provide to quickly and securely perform GMR resetting by excluding the influence of a magnetic field generated by a storage medium in a disk drive.
It is still another object of the present invention to provide a magnetic disk drive in which the above error recovery method is applied.
The present invention to recovers an operation error arising in an information recording and reproducing unit provided with a head using a GMR sensor by confirming whether the error is caused by the GMR sensor deterioration and thereafter applies a reset pulse to the GMR sensor.
According to a mode of the present invention, a first error recovery procedure is executed after an operation error occurs. The first error recovery procedure does not include application of a reset pulse. Therefore, when the operation error is caused by deterioration of the performance of a GMR sensor, the error may not be recovered by the first error recovery procedure. In this case, GMR resetting is not immediately performed but a first GMR evaluation procedure is executed to confirm whether the error is caused by the GMR sensor. According to the above structure, even if the error cannot be recovered due to a cause other than the GMR sensor, it is possible to prevent the performance of the sensor from deteriorating by performing GMR resetting and quickly execute other possible recovery means.
According to another mode of the present invention, when a reset pulse is applied and thereafter a second error recovery procedure is executed but an error still cannot be recovered, a second GMR evaluation procedure is executed and thereafter, a reset pulse is applied. According to the above structure, even if the performance of a GMR sensor cannot be recovered through the last GMR resetting, the performance of the GMR sensor can be recovered by repeating GMR resetting second time. Moreover, when it is decided in accordance with a second GMR evaluation procedure that the performance of the sensor is deteriorated due to GMR resetting, it is possible to decide that application of a reset pulse is not effective for recovery of the performance of the sensor and prevent the sensor from damaging due to careless repetition of GMR resetting.
According to still another mode of the present invention, various modes are provided as evaluation items of the first GMR evaluation procedure. Though at least one evaluation item is necessary, it is possible to improve the accuracy by combining a plurality of items. Moreover, the second GMR evaluation procedure makes it possible to improve the accuracy of performance evaluation by using the bit pattern of user data or the standard test bit pattern recorded in an exclusive calibration sector and thereby, selecting an item capable of directly measuring whether the performance of a sensor after performing GMR resetting is changed.
According to still another mode of the present invention, the number of reassignments performed under execution of an error recovery procedure is counted and GMR resetting is performed in accordance with the number of reassignments. Therefore, it is possible to prevent a state in which the access time of a unit is increased because deterioration of a GMR sensor is accelerated and the number of reassignments increased and prevent all of the substitute sectors from being used. Moreover, to confirm that the number of reassignments is increased due to a GMR sensor, it is possible to evaluate the error of the waveform of a regenerative signal and improve the effectiveness of GMR resetting. Furthermore, by checking if data can be read from or written in a reassignment source sector after performing GMR resetting, it is confirmed whether executed reassignment can be recovered. When the reassignment is caused by a GMR sensor and the performance of a head is recovered by GMR resetting, it is possible to cancel the reassignment, enlarge the number of substitute sectors, and moreover, decrease the access time for the head. Furthermore, it is also possible to use the dummy write frequency for the decision on resetting as an index showing deterioration of a GMR sensor instead of the number of reassignments.
According to still another mode of the present invention, a head is positioned to a position not influenced by the magnetic flux generated by a storage medium when applying a reset pulse to effectively execute GMR resetting. To obtain a position not influenced by magnetic flux, it is possible to position a head to an exclusive DC-erased area provided for the recording area on a magnetic disk or outermost or innermost cylinder in which no data or servo data is stored. When performing GMR resetting by stopping a unit, it is possible to position a head to a non-recording area in the case of a contact start/stop (CSS) unit or position a head to a ramp in the case of a load/unload type unit.